Carious teeth, so-called dental caries, have recently been treated with rotary cutting devices typified by air turbines or Er:YAG laser light having a wavelength of 2.94 μm. Such techniques cannot separate sound and carious parts from each other. Therefore, the treatment for dental caries tends to depend on skills of dentists. Hence, minimally invasive therapeutic techniques which can selectively process carious parts have been in demand. As one of such techniques, those of Non Patent Literatures 1 and 2 utilizing a mid-infrared wavelength-variable laser of difference-frequency generation (DFG) type (hereinafter referred to as DFG laser) have been known. The difference-frequency generation is a method which makes two kinds of wavelengths λ1, λ2 incident on a nonlinear optical crystal and satisfies a phase-matching condition, so as to generate light having a wavelength λ3. The DFG laser of Non Patent Literature 1 employs AgGaS2 (silver gallium sulfide) as a nonlinear optical crystal, and an Nd:YAG laser (wavelength λ1: 1.064 μm) and a Cr:forsterite laser (wavelength λ2: 1.15 to 1.36 μm) as DFG pumping light and signal light, respectively. This is the same in Non Patent Literature 2.